Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and an apparatus for transmitting uplink data.
Related Art
Long-Term Evolution (LTE) based on 3rd Generation Partnership Project (3GPP) Technical Specification (TS) Release 8 is a leading next-generation mobile communication standard.
As set forth in 3GPP TS 36.211 V8.7.0 (May 2009) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8),” physical channels in LTE may be classified into downlink channels, such as physical downlink shared channel (PDSCH) and physical downlink control channel (PDCCH), and uplink channels, such as physical uplink shared channel (PUSCH) and physical uplink control channel (PUCCH). The PUCCH is an uplink control channel used for transmitting uplink control information, such as a hybrid automatic repeat request (HARQ) acknowledgement (ACK)/non-acknowledgement (NACK) signal, a channel quality indicator (CQI) and a scheduling request (SR).
Meanwhile, an evolution of 3GPP LTE, LTE-Advanced (LTE-A), is developing. 3GPP LTE-A adopts carrier aggregation. Carrier aggregation uses a plurality of component carriers. A component carrier is defined by center frequency and bandwidth. In carrier aggregation, a plurality of component carriers corresponds to a single cell. A user equipment (UE) provided with a service using a plurality of downlink component carriers may be interpreted as being provided with the service from a plurality of serving cells. Uplink synchronization of a UE in the case of using carrier aggregation may be difference uplink synchronization of a UE in the case of not using carrier aggregation.
To reduce interference between UEs due to uplink transmissions, it is important for a base station to maintain uplink time alignments of the UEs. A UE may be located at a random place within a cell and an arrival time an uplink signal transmitted from a UE takes to reach the base station may vary depending on the location of each UE. A UE located at a cell edge has a longer arrival time than a UE located in a cell center. On the contrary, the UE located in the cell center has a shorter arrival time than the UE located at the cell edge.
To reduce interference between UEs, it is necessary that a base station schedules uplink signals transmitted by UEs in a cell to be received within each time boundary. The base station needs to properly adjust transmission timings of the respective UEs according to situations of the respective UEs and such adjustment is referred to as uplink time alignment. A random access process is one of processes for maintaining uplink time alignment. A UE acquires a time alignment value (also referred to as a timing advance (TA) value) through the random access process and maintains uplink time alignment by applying the time alignment value. As described above, when carrier aggregation is performed in 3GPP LTE-A, a procedure that a UE performs uplink time alignment and uplink transmission may be different from that in the case of performing no carrier aggregation.